Catalyst for isomerizing alkylaromatics

ABSTRACT

This invention presents a novel catalyst formulation for the isomerization of alkylaromatic hydrocarbons. The catalyst comprises at least one Group VIII metal, a pentasil zeolite wherein a portion of the aluminum atom has been replaced with gallium atoms and a matrix material of zirconia-alumina. When utilized in a process for isomerizing a non-equilibrium mixture of xylenes containing ethylbenzene, a greater yield of para-xylene is obtained compared to prior art processes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of prior copendingapplication Ser. No. 281,424 now U.S. Pat. No. 4,886,927, filed 12-8-88,which is a division of Ser. No. 109,019, filed 10-16-87, abandoned, thecontents of which are incorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention relates to an improved catalyst for the isomerization ofxylenes and conversion of ethylbenzene. Most specifically, the inventionconcerns a catalyst composition comprising a Group VIII metal component,a gallium-substituted pentasil zeolite, and a zirconia-alumina matrix.

BACKGROUND OF THE INVENTION

The xylene, namely ortho-xylene, meta-xylene and para-xylene, areimportant chemicals and find wide and varied application in industry.Orthoxylene is a reactant for the production of phthalic anhydride.Meta-xylene is used in the manufacture of plasticizers, azo dyes, woodpreservers, etc. Paraxylene upon oxidation yields terephthalic acidwhich is used in the manufacture of synthetic textile fibers.

As a result of the important applications to which the individual xyleneisomers are subjected, it is often very important to be able to producehigh concentrations of a particular xylene. This can be accomplished byconverting a non-equilibrium mixture of the xylene isomers, whichmixture is low in the desired xylene isomer, to a mixture whichapproaches equilibrium concentrations. Various catalysts and processeshave been devised to accomplish the isomerization process. For example,it is well known in the art that catalysts such as aluminum chloride,boron fluoride, liquid hydrofluoric acid, and mixtures of hydrofluoricacid and boron fluoride can be used to isomerize xylene mixtures.

Industrially, isomerization of xylenes and conversion of ethylbenzene isperformed primarily to produce para-xylene. A typical processing schemefor this objective comprises: (a) separating para-xylene from a C₈alkylaromatic mixture using, for example, molecular sieve technology, toobtain a para-xylene-rich stream and a para-xylene-depleted stream; (b)isomerizing the para-xylene depleted stream to near equilibrium in anisomerization reaction zone; and, (c) recycling the isomerizationproduct to separation along with the fresh C₈ alkylaromatic mixture.

The present invention is particularly concerned with the isomerizationreaction step which may be used in an overall process directed topara-xylene production. An important parameter to consider in thisisomerization reaction step is the degree of approach to xyleneequilibrium achieved. The approach to equilibrium that is used is anoptimized compromise between high C₈ aromatic ring loss at highconversion (i.e. very close approach to equilibrium) and high utilitycosts due to the large recycle rate of unconverted ethylbenzene,orthoxylene, and meta-xylene. Also contributing to the recycle streamare C₈ naphthenes which result from the hydrogenation of the C₈aromatics.

It is desirable to run the isomerization process as close to equilibriumas possible in order to maximize the para-xylene yield, however,associated with this is a greater cyclic C₈ loss due to side-reactions.Cyclic C₈ hydrocarbons include xylenes, ethylbenzene, and C₈ naphthenes.The correlation of cyclic C₈ loss versus the distance from xyleneequilibrium is a measure of catalyst selectivity. Thus there is a strongincentive to develop a catalyst formulation which minimizes cyclic C₈loss while maximizing para-xylene yield.

Numerous catalysts have been proposed for use in xylene isomerizationprocesses such as mentioned above. More recently, a number of patentshave disclosed the use of cystalline aluminosilicate zeolite-containingcatalysts for isomerization and conversion of C₈ alkylaromatics.Crystalline aluminosilicates generally referred to as zeolites, may berepresented by the empirical formula:

    M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2 yH.sub.2 O

in which n is the valence of M which is generally an element of Group Ior II, in particular, sodium, potassium, magnesium, calcium, strontium,or barium, and x is generally equal to or greater than 2. Zeolites haveskeletal structures which are made up of three-dimensional networks ofSiO₄ and AlO₄ tetrahedra, corner-linked to each other by shared oxygenatoms. Zeolites with high SiO₂ /Al₂ O₃ ratios have received muchattention as components for isomerization catalysts. Representative ofzeolites having such high proportion of SiO₂ include mordenite and theZSM varieties. It is also known in the art that zeolites of the ZSMseries can be prepared with gallium atoms substituted for aluminumatoms, for example, see U.S. Pat. No. 4,585,641. In addition to thezeolite component, certain metal promoters and inorganic oxide matriceshave been included in isomerization catalyst formulations. Examples ofinorganic oxides include silica, alumina, and mixtures thereof. Metalpromoters such as Group VIII or Group III metals of the Periodic Table,have been used to provide a dehydrogenation functionality. The acidicfunction can be supplied by the inorganic oxide matrix, the zeolite, orboth.

When employing catalysts containing zeolites for the isomerization ofalkylaromatics, characteristics such as acid site strength, zeolite porediameter, and zeolite surface area become important parameters toconsider during formulation development. Variation of thesecharacteristics in a way that reduces side-reactions, such as,transalkylation, is required in order to achieve acceptable levels ofcyclic C₈ loss.

It has been found that, if a catalyst is formulated with the components,and in the manner set forth hereinafter, an improved process for theconversion of a non-equilibrium mixture of xylenes containingethylbenzene is obtained.

OBJECTS AND EMBODIMENTS

A principal object of the present invention is to provide a novelcatalyst for the isomerization of isomerizable hydrocarbons. Morespecifically, the instant invention is aimed at a catalyst compositionwhich, when utilized for the isomerization of alkylaromatichydrocarbons, results in minimal loss of the alkylaromatic hydrocarbons.Other objects of the instant invention are to present a method ofpreparation and a process use of the catalyst.

Accordingly, a broad embodiment of the invention is directed toward acatalyst for the isomerization of isomerizable hydrocarbons comprisingat least one Group VIII metal component, a gallium-substituted pentasilzeolite, and a zirconia-alumina matrix. A preferred pentasil zeolite isa zeolite having an x-ray diffraction pattern equivalent to that ofZSM-5.

Another embodiment is directed toward a process for the isomerization ofa feed stream comprising a non-equilibrium mixture of xylenes containingethylbenzene, which comprises contacting the feed in the presence ofhydrogen at a temperature of from about 300° to 500° C., a pressure offrom about 69 to about 6895 kPa (ga), a liquid hourly space velocity offrom about 0.5 to about 10 hr⁻¹ with a catalyst comprising at least oneGroup VIII metal component, a gallium-substituted pentasil zeolite, anda zirconia-alumina matrix.

These as well as other objects and embodiments will become evident fromthe following more detailed description of the invention.

INFORMATION DISCLOSURE

The prior art recognizes numerous isomerization processes employing avariety of catalyst formulations. However, it is believed that none ofthe prior art processes recognizes the use of the catalyst formulationand method of making same which forms an integral part of the instantinvention.

U.S. Pat. No. 3,923,639 (Ciric) is directed to a hydrocarbon crackingprocess utilizing a catalyst composition comprising a crystallinealuminosilicate ZSM-4 zeolite. Although the reference lists as possiblecomponents Group VIII metals and a variety of matrix materials,including alumina-zirconia, the reference is silent as to the utility ofa gallium-substituted pentasil in combination with a Group VIII metaland zirconia-alumina matrix for the isomerization of alkylaromatichydrocarbons.

The conversion of heavy reformate using a variety of different catalystcompositions, including silica-alumina containing pentasil zeolites, istaught in U.S. Pat. No. 4,066,531 (Owen et al). The utility of zirconiain combination with clay, alumina and silica is recognized as a suitablebinding material. However, the reference is not cognizant of the utilityof a gallium-substituted pentasil zeolite in combination with the othercomponents of the instant invention.

U.S. Pat. No. 4,255,288 (Cull et al) teaches a catalyst comprising aY-type zeolite, alumina, zirconia, and at least one each of Group VIBand Group VIII metals. Hydrocracking and hydrodesulfuization tests showsuperior results for catalysts of the invention. The reference does notdisclose a gallium substituted pentasil zeolite.

Several relevant references are directed to processes and catalystcompositions specifically for isomerizing alkylaromatics. Related U.S.Pat. Nos. 4,331,822 and 4,485,185 (Onodera et al) teach the use of acatalyst containing silica-alumina pentasil zeolites having addedthereto platinum and a second metal. However, neither referencerecognizes gallium-substituted pentasil zeolites nor the use of azirconia-alumina matrix. U.S. Pat. No. 4,482,773 (Chu et al) is directedto a process for isomerizing a mixture of xylenes and ethylbenzene witha ZSM-5 catalyst containing platinum and a Group IIA component. However,the reference does not recognize the utility of gallium substitution inthe zeolite or of a zirconia-alumina matrix. Another reference, U.S.Pat. No. 4,584,423 (Nacamuli et al), teaches a process for isomerizing anon-equilibrium mixture of xylenes containing ethylbenzene in theabsence of hydrogen using a catalyst containing a pentasil zeolitewherein gallium may be substituted for aluminum. This reference makes nomention of the utility of either a zirconia-alumina matrix nor theincorporation of a Group VIII metal.

U.S. Pat. No. 4,599,475 (Kresge et al.) teaches an isomerization processusing a catalyst comprising ZSM-23 zeolite. Kresge et al. disclosegallium among a broad range of nine "Y" framework elements and aluminaand zirconia amoung a non-limiting list of seven binder materials.However, Kresge et al. teach away from the use of the ZSM-5 of thepresent catalyst in the disclosed range of framework and binderelements.

In summary, it appears that the prior art only generally recognizes thatzeolite have utility for isomerization of isomerizable alkylaromaticsand that no single reference teaches nor suggests the invention claimedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically compares isomerization performance of Catalysts A andB of the invention and Catalyst C of the prior art, relating the lost ofC₈ cyclic hydrocarbons due to side reactions in anisomerization/separation process combination to the paraxylene contentof the xylenes product.

FIG. 2 compares the performance of Catalysts A and B of the inventionwith that of Catalyst C of the prior art, relating para-xylene yield tothe ratio of C₈ cyclics recycled to the isomerization reactor in anisomerization/separation process combination.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention is concerned with a catalystcomposition useful for the isomerization and conversion of anon-equilibrium mixture of C₈ aromatic hydrocarbons. This catalyticcomposite comprises at least one Group VIII metal component, a pentasilzeolite wherein a portion of aluminum atoms have been replaced withgallium atoms and a matrix material comprising zironia-alumina. Whenutilized in a process for isomerizing a nonequilibrium mixture ofalkylaromatics, the instant invention allows for a closer approach toxylene equilibrium resulting in a greater yield of para-xylene withoutthe high loss of C₈ aromatics common to prior art processes.

The catalyst of the instant invention contains at least one Group VIIImetal component of the Periodic Table (see, Cotton and Wilkinson,Advanced Inorganic Chemistry (3rd Ed., 1972)). Preferably, this GroupVIII metal is selected from the platinum group metals. Of the platinumgroup metals, which include palladium, rhodium, ruthenium, osmium andiridium, the use of platinum is preferred. The platinum group componentmay exist within the final catalyst composite as a compound such as anoxide, sulfide, halide, oxysulfide, etc., or as an elemental metal or incombination with one or more other ingredients of the catalyst. It isbelieved that the best results are obtained when substantially all theplatinum group component exists in the elemental state. The platinumgroup component generally comprises from about 0.01 to about 2 wt. % ofthe final catalytic composite, calculated on an elemental basis. It ispreferred that the platinum content of the catalyst be between about 0.1and 1 wt. %. The preferred platinum group component is platinum, withpalladium being the next preferred metal. The platinum group componentmay be incorporated into the catalyst composite in any suitable mannersuch as by coprecipitation or cogelation with the zirconia-aluminamaterial, or by ion-exchange or impregnation of the zeolite, and byion-exchange or impregnation of the zeolite and zirconia-aluminacomposite. The preferred method of preparing the catalyst normallyinvolves the utilization of a water-soluble, decomposable compound of aplatinum group metal to impregnate the zeolite and zirconia-aluminacomposite. For example, the platinum group component may be added to thecomposite by commingling the composite with an aqueous solution ofchloroplatinic or chloropalladic acid. An acid such as hydrogen chlorideis generally added to the impregnation solution to aid in thedistribution of the platinum group component through the compositeparticles.

After addition of the Group VIII metal component to the zeolite andzirconia-alumina composite, the resultant composite is dried at atemperature ranging from about 100° to about 200° C. for a period of atleast 2 to about 24 hours or more, and finally calcined or oxidized at atemperature ranging from about 450° to about 650° C. in air or oxygenatmosphere for a period of about 0.5 to about 10 hours in order toconvert all of the metallic components to the corresponding oxide form.The resultant oxidative composite is preferably subjected to asubstantially water-free reduction step prior to its use in theisomerization of hydrocarbons. This step is designed to selectivelyreduce the Group VIII metal component to the elemental metallic stateand to ensure a uniform and finely divided dispersion of the metalliccomponent throughout the catalyst. Preferably, a substantially pure anddry hydrogen stream (i.e. less than 20 vol. ppm H₂ O) is used as thereducing agent in this step. The reducing agent is contacted with theoxidized catalyst at conditions including a reduction temperatureranging from about 200° to about 650° C. and a period of time of about0.5 to 10 hours effective to reduce substantially all of the Group VIIImetal component to the elemental metallic state.

The resulting reduced catalytic composite may, in some cases, bebeneficially subjected to a presulfiding operation designed toincorporate in the catalytic composite from about 0.05 to about 0.5 wt.% sulfur calculated on an elemental basis. Preferably, this presulfidingtreatment takes place in the presence of hydrogen and a suitablesulfur-containing compound such as hydrogen sulfide, lower molecularweight mercaptans, organic sulfides, etc. Typically, this procedurecomprises treating the reduced catalyst with a sulfiding gas such as amixture of hydrogen and hydrogen sulfide having about 10 moles ofhydrogen per mole of hydrogen sulfide at conditions sufficient to effectthe desired incorporation of sulfur, generally including a temperatureranging from about 10° up to about 593° C. or more. It is generally agood practice to perform this presulfiding step operation undersubstantially water-free conditions.

The gallium-substituted pentasil zeolite utilized in the instantinvention preferably has a formula (expressed in terms of mole ratios ofoxides) as follows:

    M.sub.2/n O:W.sub.2 O.sub.3 :ySiO.sub.2 :zH.sub.2 O

wherein M is at least one cation of valence n, W is gallium and/oraluminum, y is at least 5, preferably at least 12, and z is from 0 to40. The zeolite preferably has an X-ray diffraction characteristic ofpentasil zeolites, which includes ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-23,and ZSM-35, with ZSM-5 being particularly preferred. "Pentasil" is aterm used to describe a class of shape-selective zeolites. This novelclass of zeolites is well known to the art and is typicallycharacterized by a silica/alumina mole ratio of at least about 12.Suitable descriptions of the pentasils may be found in U.S. Pat. Nos.4,159,282; 4,163,018; and 4,278,565, all of which are incorporatedherein by reference. The zeolite framework may contain only gallium andsilicon atoms or may contain a combination of gallium, aluminum, andsilicon atoms. The gallium content, expressed as mole ratios of SiO₂/Ga₂ O₃, may range from 20:1 to 400:1. The preferred gallium-substitutedpentasil zeolite has a ZSM-5 structure with a gallium content rangingfrom 0.1 to 10 wt. % of the zeolite, most preferably ranging from 0.5 to5 wt. %. The gallium-substituted pentasil zeolite may be prepared bycrystallization from a reaction mixture comprising a silica source, asource of Ga₂ O₃, a source of Al₂ O₃ if desired, and optionally anorganic template compound. It is believed that the preparation ofzeolites is within the competence of one skilled in the art and aparticular preparation method is not critical to the instant invention.It is preferred that the catalyst of the instant invention contain from1 to 20 wt. % gallium-substituted ZSM-5 zeolite. In a preferredembodiment, the catalyst comprises 1 to 20 wt. % of gallium-substitutedZSM-5 zeolite and 80 to 99 wt. % zirconia-alumina matrix.

In accordance with the present invention, the catalyst contains azirconia-alumina matrix. This matrix is a composite of two porousrefractory inorganic oxides having basic chemical formulae of ZrO₂ andAl₂ O₃, respectively. Suitable alumina materials are the crystallinealuminas known as gamma-, eta-, and theta-, with gamma- or eta-aluminabeing the most preferred. It is preferred that the matrix contains fromabout 90 to about 99 wt. % alumina. The zirconia portion of the matrixpreferably constitutes frm about 1 to about 10 wt. % of the matrix.Preferred physical properties of the matrix include an apparent bulkdensity of 0.3 to about 0.8 g/cc and surface area characteristics suchthat the average pore diameter is about 20 to 300 angstroms, the porevolume is about 0.1 to about 1 cc/g, and the surface area is about 100to 500 m² /g.

Preparation of the matrix material may be performed in any suitablemanner known to the art. A particularly preferred method of preparingthe zirconia-alumina matrix is belived to result in a finished catalystthat exhibits superior performance when utilized for the conversion ofhydrocarbons. This preferred method involves cogelation of zirconia andalumina in an intimate admixture with the gallium-substituted pentasilzeolite. The first step in the preparation method involves the formationof an alumina hydrosol. Any technique known to the art may be utilizedto prepare the alumina hydrosol, however, a preferred method involvesreacting aluminum metal with hydrochloric acid. The gallium-substitutedpentasil zeolite is then added to the alumina hydrosol to form ahomogeneous mixture. The amount of zeolite added is dependent on theultimate end use of the finished catalyst. A preferred matrix/zeoliteweight ratio in the finished catalyst ranges from 4:1 to 99:1, with amore preferred weight ratio range of 9:1 to 19:1. To the alumina sol andzeolite mixture is added a zirconia sol, for example, zirconiumoxychloride, and to the resultant mixture is added a suitable gellingagent, such as, hexamethylenetetramine. It is believed that the order ofcombining the alumina sol, zirconia sol, zeolite, and gelling agent isunimportant. Therefore, any combination sequence of the ingredientsshould produce the catalyst of the instant invention. Once gelled, thecomposite may be formed into any desired shape such as spheres, pills,cakes, extrudates, powders, granules, tablets, etc. and utilized in anydesired size. In a preferred embodiment, the resultant mixture is firstshaped in the form of a sphere and then gelled.

For purposes of the present invention, a particularly preferred shape ofthe composite is a sphere, continuously manufactured by the well-knownoil drop method. In summary, this method involves dropping the mixtureof zeolite, alumina sol, zirconia sol, and gelling agent into an oilbath maintained at elevated temperatures. The droplets of the mixtureremain in the oil bath until they set and form hydrogel spheres. Thespheres are then continuously withdrawn from the oil bath and typicallysubjected to specific aging treatments in oil and an ammoniacal solutionto further improve their physical characteristics. The resulting agedand gelled particles are then washed and dried at a relatively lowtemperture of about 50°-200° C. and subjected to a calcination procedureat a temperature of about 450°-700° C. for a period of about 1 to about20 hours. This treatment effects conversion of the hydrogel to thecorrespondign zirconia-alumina matrix. In a preferred embodiment, thecalcined composite is washed to remove any remaining alkali metalcations that may be present. The wash solution is preferably an aqueousammonium solution, most preferably containing about 0.5% NH₃ in water.After washing at about 95° C., the composite is dried at about 110° C.See the teachings of U.S. Pat. No. 2,620,314 for additional details.

The process of this invention is applicable to the isomerization ofisomerizable alkylaromatic hydrocarbons of the general formula:

    C.sub.6 H.sub.(6-n) R.sub.n

where n is an integer from 2 to 5 and R is CH₃, C₂ H₅, C₃ H₇, or C₄ H₉,in any combination and including all the isomers thereof. Suitablealkylaromatic hydrocarbons include, for example, ortho-xylene,meta-xylene, para-xylene, ethylbenzene, ethyltoluenes, thetrimethylbenzenes, the diethylbenzenes, the triethylbenzenes,methylpropylbenzenes, ethylpropylbenzenes, the diisopropylbenzenes, thetriisopropylbenzenes, etc., and mixtures thereof.

It is contemplated that any aromatic C₈ mixture containing ethylbenzeneand xylene may be used as feed to the process of this invention.Generally, such mixture will have an ethylbenzene content in theapproximate range of 5 to 50 wt. %, an ortho-xylene content in theapproximate range of 0 to 35 wt. %, a meta-xylene content in theapproximate range of 20 to 95 wt. % and a para-xylene content in theapproximate range of 0 to 15 wt. %. It is preferred that theaforementioned C₈ aromatics comprise a non-equilibrium mixture. The feedto the instant process, in addition to C₈ aromatics, may containnonaromatic hydrocarbons, i.e. naphthenes and paraffins in an amount upto 30 wt. %.

The alkylaromatic hydrocarbons for isomerization may be utilized asfound in selective fractions from varous refinery petroleum steams,e.g., as individual components or as certain boiling range fractionsobtained by the selective fractionation and distillation ofcatalytically cracked gas oil. The process of this invention may beutilized for conversion of isomerizable aromatic hydrocarbons when theyare present in minor quantities in various streams. The isomerizablearomatic hydrocarbons which may be used in the process of this inventionneed not be concentrated. The process of this invention allows theisomerization of alkylaromatic containing streams such as reformate toproduce specified xylene isomers, particularly para-xylene, thusupgrading the reformate from its gasoline value to a high petrochemicalvalue.

According to the process of the present invention, an alkylaromatichydrocarbon charge stock, preferably in admixture with hydrogen, iscontacted with a catalyst of the type hereinafter described in analkylaromatic hydrocarbon isomerization zone. Contacting may be effectedusing the catalyst in a fixed bed system, a moving bed system, afluidized bed system, or in a batch-type operation. In view of thedanger of attrition loss of the valuable catalyst and of operationaladvantages, it is preferred to use a fixed bed system. In this system, ahydrogen-rich gas and the charge stock are preheated by suitable heatingmeans to the desired reaction temperature and then passed into anisomerization zone containing a fixed bed of catalyst. The conversionzone may be one or more separate reactors with suitable meanstherebetween to ensure that the desired isomerization temperature ismaintained at the entrance to each zone. It is to be noted that thereactants may be contacted with the catalyst bed in either upward,downward, or radial flow fashion, and that the reactants may be in theliquid phase, a mixed liquid-vapor phase, or a vapor phase whencontacted with the catalyst.

The process of this invention of isomerizing an isomerizablealkylaromatic hydrocarbon is preferably effected by contacting thealkylaromatic, in a reaction zone containing an isomerization catalystas hereinafter described, with a fixed catalyst bed by passing thehydrocarbon in a down-flow or radial flow fashion through the bed, whilemaintaining the zone at proper alkylaromatic isomerization conditionssuch as a temperature in the range from about 0°-600° C. or more, and apressure of about 101 kPa (abs) to about 10,340 kPa (ga) or more.Preferably, the operating temperature ranges from about 300°-500° C. andthe pressure ranges from about 69 to about 6,895 kPa (ga). Thehydrocarbon is passed, preferably, in admixture with hydrogen at ahydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1 or more,and at a liquid hourly hydrocarbon space velocity of about 0.1 to about20 hr⁻¹ or more, most preferably at 0.5 to 10 hr⁻¹. Other inert diluentssuch as nitrogen, argon, etc., may be present.

The particular product recovery scheme employed is not deemed to becritical to the instant invention. Any recovery scheme known in the artmay be used. Typically, the reactor effluent will be condensed with thehydrogen and light hydrocarbon components removed therefrom by flashseparation. The condensed liquid product is then subject to afractionation procedure to further purify the desired liquid product. Insome instance, it may be desirable to recover certain product species,such as ortho-xylene, by selective fractionation. In most instances, theliquid xylene product is processed to selectively recover thepara-xylene isomer. Recovery of para-xylene can be performed bycrystallization methods or most preferably by selective adsorption usingcrystalline aluminosilicates.

The following examples are presented for purpose of illustration onlyand are not intended to limit the scope of the present invention.

EXAMPLES

The examples present test results obtained when catalysts of theinvention were evaluated in an isomerization process. The catalysts wereevaluated using a pilot plant flow reactor processing a non-equilibriumC₈ aromatic feed comprising 52.0 wt. % meta-xylene, 18.5 wt. %ortho-xylene, 0.1 wt. % para-xylene, 21.3 wt. % ethylbenzene, and 0.1wt. % toluene, with the balance being nonaromatic hydrocarbons. Thisfeed was contacted with 100 cc of catalyst at a liquid hourly spacevelocity of 2, and a hydrogen/hydrocarbon mole ratio of 4. Reactorpressure and temperature were adjusted to cover a range of conversionvalues in order to develop the relationship between C₈ ring loss andapproach to xylene equilibrium (as determined by product para-xylene tototal xylene weight ratio). At the same time, at each temperature, thepressure was chosen to maintain a constant mole ratio of C₈ naphthenesto C₈ aromatics of approximately 0.06.

EXAMPLE I

A quantity of gallium-substituted pentasil zeolite having an X-raydiffraction pattern equivalent to that of ZSM-5 was prepared by adding asilica source, Ludox HS-40, to an aqueous solution containing an organictemplate, tetrapropylammonium bromide. The weight ratio of silica totemplate was about 1:1. A solution of sodium gallate was added to thesilica and template mixture in an amount to give about 1.0 wt. % galliumbased on the finished zeolite. The resultant mixture was autoclaved atabout 150° C. for approximately 140 hours. The zeolite obtained waswashed, filtered and dried to yield a gallium-substituted pentasilzeolite containing approximately 1.3 wt. % Ga.

A portion of the zeolite described above was mixed with aluminahydrosol, prepared by digesting metallic aluminum in hydrochloric acid,to yield a zeolite content in the finished catalyst equal to about 10wt. %. To this mixture was added enough zirconium oxychloride sol,containing approximately 20 wt. % ZrO₂, such that the finished zeolitezirconia-alumina composite contained approximately 5 wt. % ZrO₂.Finally, a solution of hexamethylenetetramine was added as a gellingagent. The final mixture was dispersed as droplets into an oil bath at atemperature of about 95° C. The droplets remained in the oil bath untilthey formed hydrogel spheres. The spheres were removed from the oil bathand washed with an aqueous solution containing about 0.5 wt. % ammonia.The spheres were then air dried at 110° C. for about 12 hours and thencalcined in air at a temperature of about 650° C. After calcination, thecomposite was washed with 0.5% NH₃ /H₂ O solution at 95° C. and thenoven-dried at 110° C.

The dried spheres were next impregnated with a solution ofchloroplatinic acid, containing 2 wt. % hydrochloric acid (based on thecalcined spheres), to yield a final platinum concentration of 0.28 wt.%. The impregnated spheres were oxidized and chloride adjusted at 525°C., reduced in molecular hydrogen at 565° C., and then sulfided withhydrogen sulfide at ambient temperature to a target sulfur level of 0.1wt. %. The finished catalyst was designated "Catalyst A".

In FIG. 1, the X-axis is the concentration of para-xylene in theproduct, expressed as mole percent relative to the total xylenes in theproduct. The Y-axis represents the amount of C₈ cyclic hydrocarbons lostdue to side reactions. This parameter is defined as the sum of C₈aromatics and naphthenes in the feed minus the amount of C₈ aromaticsand naphthenes in the product divided by the C₈ aromatics and naphthenesin the feed.

EXAMPLE II

Catalyst B of the invention and Catalyst C of the prior art wereprepared using substantially the same methods as described in Example I,with exceptions as noted hereinbelow.

Sufficient zeolite was mixed with alumina hydrosol to yield a zeolitecontent in each of finished Catalysts B and C of about 4.5 wt. %. Thezirconium oxide sol was omitted in the preparation of Catalyst C.

Each of the composites after oven-drying were reoxidized in dry air atabout 565° C. Impregnation was carried out with a solution ofchloroplatinic acid containing about 4 wt. % HCI. The oxidized,chloride-adjusted and sulfided spheres had the following approximateanalysis:

    ______________________________________                                                         Catalyst B                                                                            Catalyst C                                                            (Invention)                                                                           (Prior Art)                                          ______________________________________                                        Apparent bulk density, g/cc                                                                      0.511     0.590                                            Platinum, wt. %    0.36      0.29                                             Chloride, wt. %    0.67      0.79                                             Sulfur, wt. %      0.08      0.07                                             ______________________________________                                    

Note that the platinum contents were adjusted in relation to apparentbulk density to be well within a 10% variation on a volume basis forexperimental purposes.

EXAMPLE III

The performance of Catalysts A and B of the invention and Catalyst C ofthe prior art for isomerization of xylenes and ethylbenzene was comparedusing essentially the same non-equilibrium C₈ aromatic feed and testconditions described hereinabove. The performance was compared usingcombined isomerization and separation processes as describedhereinabove, with C₈ -aromatic isomers being recycled until they areconverted to either recovered para-xylene or to lighter or heavierbyproducts.

The results were compared with respect to the loss of valuable C₈aromatic rings to byproducts and with respect to the overall yield ofparaxylene. FIG. 1 shows C₈ ring loss as a function of paraxylene in thetotal xylene stream. Paraxylene in total xylenes represents the approachto equilibrium paraxylene content, a measure of xylene-isomerizationseverity. At each level of xylene-isomerization severity, C₈ ring lossis significantly lower for Catalyst B of the invention than the CatalystC of the prior art.

FIG. 2 shows a comparison of para-xylene yield relative to recycle ratiofor Catalysts A and B of the invention and Catalyst C of the prior art.Recycle ratio is defined as the weight ratio of the C₈ A+C₈ N productrecycle recycled from the isomerization reaction zone to the fresh C₈ Afeed addition to the process. Low recycle ratios require lower utilityand capital costs and are commerically desirable. The para-xylene yieldis defined as the weight percentage of the fresh C₈ A feed that isultimately converted to the desired product para-xylene stream.Representation of catalyst performance data this way had the advantagethat ethylbenzene conversion as well as C₈ ring loss and approach topara-xylene equilibrium are combined into a single graph. EB conversionin an isomerization loop is relatively difficult, and it readily buildsup in the separation/isomerization loop. High-conversion catalystsprovide lower recycle ratios while improved selectivity shows as anincreased para-xylene yield. As is shown in FIG. 2, both Catalysts A andB of the invention show favorably higher para-xylene yield at a givenrecycle ratio than does the prior art Catalyst C.

We claim:
 1. A catalyst for the isomerization of a non-equilibrium C₈-aromatic mixture containing xylenes and ethylbenzene comprising atleast one platinum group metal and a gallium-substituted pentasilzeolite having a ZSM-5 structure in a zirconia-alumina matrix containingfrom about 1 to 10 wt. % zirconia, the catalyst having a matrix/zeoliteweight ratio of 9:1 to 19:1.
 2. The catalyst of claim 1 furthercharacterized in that the platinum group metal component comprises fromabout 0.1 to 5 wt. % platinum.
 3. The catalyst of claim 1 furthercharacterized in that the zeolite contains from about 0.1 to 10 wt. %gallium.
 4. The catalyst of claim 1 further characterized in that thecatalyst comprises 0.05 to about 0.5 wt. % sulfur calculated aselemental sulfur.